Field of the Invention
The present invention relates generally to a gas turbine engine, and more specifically to an air riding seal with a purge cavity in the turbine section of the engine.
Description of the Related Art Including Information Disclosed Under 37 CFR 1.97 and 1.98
In a gas turbine engine, compressed air from a compressor is burned with a fuel in a combustor to produce a hot gas stream that is then passed through a turbine to drive the compressor and, in the case of an industrial gas turbine engine, to also drive an electric generator to produce electrical power. The turbine typically includes several stages or rows of stator vanes and rotor blades. The stator therefore must be sealed from the rotor in order to prevent the hot gas from leaking into sections of the engine that must be protected from the high temperatures.
Labyrinth seals are the current choice of design for a seal between the rotor and the stator to prevent the hot gas from leaking into areas such as the rim cavity. A labyrinth seal (lab seal) typically will include a number of lab seal teeth extending from a rotor that forms a small gap with a surface on the stator.
One problem with lab seals in a gas turbine engine is that the gap between the teeth and the stator surface can change due to temperature changes in the turbine. Significant hot gas leakage not only decreases performance, but shortens part life. This is one reason why a honeycomb structure is used on the stator surface for the lab seal teeth to rub into. The teeth can rub without wearing out the teeth surface or the stator surface while minimizing any gap.
One improvement over the labyrinth seal in a turbine is the air riding seal disclosed in U.S. Pat. No. 8,066,473 issued to Aho J R on Nov. 29, 2011 (incorporated herein by reference). An air riding seal includes an annular piston that floats or rides over a rotor surface using a cushion of pressurized air. A near-perfect seal is formed between due to a very small gap formed between the rotor and the stator that is filled with pressurized air that prevents any leakage flow across the seal.
One of the challenges associated with the operation of the Aho air riding seal is that the leakage through one of the sealing lands can be so low that it leads to increased fluid temperature due to windage. This can lead to heat-up in the metallic seal, causing the metallic seal to expand and either rub or increase the leakage beyond what is desired. It can also lead to a much shorter seal life than anticipated. This low leakage is caused by the small pressure differential between the pocket pressure (cushion or pocket chamber 16 in the face of the annular piston 12 in the Aho patent) and the high pressure leakage cavity (19 in the Aho patent).